Summary: In the Oudin serum agar technique, at sufficiently low antigen (Ago) and high antibody (Ab), Oudin showed empirically that the equation k = m (log Ago - log R′ Ab) holds. In this equation, k is the slope of the straight line obtained by plotting x, the distance which the leading edge of the band moves in time, t, against t½. An equation of this same form had been derived previously in which m was equal to 4.08 √DA. This led to the belief that m might be proportional to √D where D is the diffusion coefficient of the antigen. This possibility was confirmed when it was shown that the increase of nonspecific protein affected m in the same way that an increase of nonspecific protein would affect D. In addition a direct proportionality between m and √D for egg albumin, human serum albumin, bovine γ-globulin, thyroglobulin and hemocyanin was demonstrated. Some possible uses for and limitations of the determination of m were briefly discussed.